I'm enjoying airtrack very much, it adds much to the immersion. However, after about one hour of flying the heading shown in airtrack is wrong, for example the FSX GPS shows 187 degrees, while airtrack shows 195 degrees. All other data are more or less correctly sent to the iPad.

Any idea about this?

Where are you reading the heading, in the MFD or the PFD? If in the MFD in which mode are you in, APP/VOR or MAP, and what does it read, TRK or HDG?

If you are in the MFD with MAP mode showing TRK and FSX is showing HDG, or if you are in APP/VOR mode showing HDG and FSX is showing tracking, it may be that the difference is just this, heading versus tracking.

When in APP or VOR mode the AirTrack's MFD shows HDG, heading, the direction the plane is facing.

When in MAP mode AirTrack shows TRK, tracking, the direction of flight.

Depending on different factors such as your aircraft's weight / mass and the wind direction and speed, the two may differ considerably. For instance if you're flying a light Cessna and have strong side winds, you may be heading 187 (HDG) and actually flying 195 (TRK) degrees for example or the difference may be even higher.

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Another possibility is the North setting under settings; is it set to magnetic or real?

I believe that FSX will give magnetic headings in most planes, so if you select real in AirTrack and if you are flying in a region of the globe where the magnetic variation is significant, say 8 degrees, that might also explain the difference. In this case just set North to Magnetic in AirTrack and they should match.

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If none of this is the cause it may be a bug; let us know and we'll try to replicate and fix it. But you say that it only occurs 1 hour after flying? This is indeed strange as if you got a delayed heading you'd get everything delayed which I presume is not the case, and the plugin should send several updates per second which would overwrite any heading that had been previously received.

hope this helps,
joao